Road signs, information boards, and reflectors for safety and security are provided with reflecting sheets or plates (collectively referred to as "reflecting sheet" hereinafter), which employ glass beads or corner-cube prisms as the reflecting elements. Unfortunately, these reflecting elements have drawbacks in their reflecting characteristics as explained below. They do not meet industrial standards if they are used as such for reflecting sheets.
The disadvantage of glass beads as reflecting elements is that they do not accomplish complete reflection on account of spherical aberration inherent in spherical lens. FIG. 12 shows the optical paths of incident rays and reflected rays of a glass bead (3) whose focal length is equal to its curvature radius. Judging from the focal length, the luminous flux of all incident rays should converge at point P on the reflection surface of the bead (3). In actual, however, this is not the case due to spherical aberration, as shown in the diagram of light paths. The resulting reflection is diffuse reflection, with incident rays reflected in all directions. This is the reason for the low luminance of reflected rays from glass beads. Therefore, the conventional reflecting sheets do not fully meet requirements for luminance.
The disadvantage of corner-cube prisms as reflecting elements is that their reflection with high luminance is highly directional. This is due to the fact that incident rays are reflected twice in the prism and there is very little difference between the angle of incidence and the angle of reflection. In other words, the reflected rays from the prism are visible only for a person standing close to the light source, and the luminance of reflected rays is very low at the optical path deflected (by several degrees) from the light source. Moreover, the fact that the luminance of reflected rays is extremely high for a person standing near the light source makes corner-cube prisms unsuitable for use on information boards. (In other words, information boards with corner-cube prisms would be dazzling for observers near the light source.)
For this reason, reflecting sheets provided with corner-cube prisms as reflecting elements do not meet the industrial standards (such as JIS) which stipulate the luminance at different angles of observation, and hence they are seldom used for road signs and information boards.
It is an object of the present invention to provide a reflecting sheet which permits reflection with higher luminance than the conventional reflecting sheets with glass beads and which fully meets various industrial standards.